$A$ thin convex lens of focal length $10 \ cm$ is placed in contact with a concave lens of the same material and of the same focal length. The focal length of the combination will be:

The focal length (in $cm$) of a lens with dispersive power $0.45$ that should be placed in contact with a convex lens of focal length $84 \,cm$ and dispersive power $0.21$ to form an achromatic combination is:

$A$ lens of power $+2 \text{ D}$ is placed in contact with a lens of power $-1 \text{ D}$. The combination will behave like:

$A$ diverging lens with a focal length magnitude of $25\ cm$ is placed at a distance of $15\ cm$ from a converging lens with a focal length magnitude of $20\ cm$. $A$ beam of parallel light falls on the diverging lens. The final image formed is:

$A$ lens of power $+2$ diopters is placed in contact with a lens of power $-1$ diopter. The combination will behave like

$A$ convex lens of focal length $100 \, cm$ and a concave lens of focal length $10 \, cm$ are placed coaxially at a separation of $90 \, cm$. If a parallel beam of light is incident on the convex lens,then after passing through the two lenses,the beam: